Generate a random string with length inherited from variable

I am working on a cryptography console application (C#) and want to include an option for the Vigenere Cipher during the encryption process to generate a One-Time Pad key that is completely random letters(A-Z) but the same length (in characters) of the message to be encrypted. So the question is how do I generate a random string with the same length of previous user input (Stringbuilder s)? I apologize in advance if this is an obvious solution. The following is my code for initializing and displaying the results of the encryption process. The nested "if" is where I want to generate the One-Time Pad key if the user enters "gen-otp":